dmaengine: stm32-dma: use platform_get_irq()

platform_get_resource(pdev, IORESOURCE_IRQ) is not recommended for
requesting IRQ's resources, as they can be not ready yet. Using
platform_get_irq() instead is preferred for getting IRQ even if it was
not retrieved earlier.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Reviewed-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>